It is important to include Electro-Static Discharge (ESD) protection in circuits or systems that may be vulnerable to damage caused by an ESD event. For example, a test instrument may be damaged if a probe touches a Device Under Test (DUT) having a different relative voltage than the test instrument. The damage is caused by a sudden rush of electrical current or a high voltage difference being discharged. To prevent damage due to ESD, many test systems include some form of ESD protection.
One form of ESD protection is found in a slotted coaxial cable design having a pair of Schottky diodes. FIG. 1 illustrates the general internal structure of such a device. A semi-rigid coaxial cable 110 generally includes a center conductor 120 separated from an electrically conductive ground shell 130 by an insulating material 140. Commonly, the insulating material 140 is formed from Teflon®, in either solid or wound-tape form. A groove or slot 150 is formed in the cable 110 deep enough to expose a portion of the center conductor 120. Then, as illustrated in FIG. 2, a pair of Schottky diodes 62, 64 are coupled between the center conductor 120 and ground shell 130. The Schottky diodes 62, 64 shunt current flow of either polarity during an ESD event, which prevents voltage disparity between components of the system, thereby protecting the components.
Some systems need to shunt more current during an ESD event than the Schottky diodes in FIG. 2 are capable of shunting. To address this, the groove 150 was widened to accommodate additional diodes so that all of the diodes could operate in their safe range. Widening the groove 150, however, reduced the performance of the test system, and specifically reduced the measuring performance at frequencies within the test band. This means that test systems suffer from either inadequate ESD protection or suffer limitations in the highest frequency that they can accurately measure.
Embodiments of the invention address these and other limitations of the prior art.